Global Navigation Satellite Systems (GNSS) such as the U.S. NAVSTAR Global Positioning System (GPS), the European Galileo system, the Chinese Beidou system, and the Russian GLONASS system are increasingly relied upon to provide synchronized timing that is both accurate and reliable. (Reference is made to GPS below, by way of example and simplicity, but similar characteristics and principles of operation apply to other GNSS.) GPS antennas are used to receive GPS signals and provide those signals to a GPS receiver. GPS antennas may amplify and filter the received GPS signals prior to passing them to the GPS receiver. The GPS receiver can then calculate position, velocity, and/or time from the signals collected by the GPS antenna. GPS timing antennas at fixed sites are susceptible to unintentional interference, such as out-of-band and multipath signals, as well as intentional interference from ground-based GPS jammers commonly employed to deny, degrade, and/or deceive GPS derived position and time.
Accurate GPS-based navigation and timing systems rely on receiving signals from at least four GPS satellites simultaneously. GPS timing systems can provide time when a single GPS satellite is observed if the position of the antenna is already known. Analysis has shown that a GPS timing antenna with a half power beam width (HPBW) of 60 degrees will have access at least 3 satellites 95% of the time, which is sufficient for timing applications. GPS satellites transmit right-hand circularly polarized (RHCP) signals, and thus, GPS antennas must be right-hand circularly polarized.
Microstrip patch antennas are often used in GPS applications due to their compact structure, light weight, and low manufacturing cost. Several types of antennas have been previously developed to mitigate interference while maintaining a sufficient RCHP HPBW for GPS applications, such as large antenna arrays, the horizon ring nulling antennas, and shorted annular ring antennas. Many of these steer a null (local gain minimum) in the direction from which interfering signals are received (such as the horizon). For example, large antenna arrays such as controlled reception pattern antennas (CRPA), steer a null in the direction of the interference using active circuitry. While CRPAs can achieve exceptional nulling in a particular direction, they can be large due to the multiple antenna elements necessary for null steering, are typically expensive due to the required active electronics, and can only null a finite number of interfering signals.
Horizon ring nulling (HRN) antennas, as described in U.S. Pat. No. 6,597,316, which is incorporated herein in its entirety, can achieve a measured RHCP null depth (i.e., zenith-to-horizon gain ratio) of approximately −45 dB on average around the entire azimuth. The HRN is composed of a shorted annular ring patch, such as that described in V. Gonzalez-Posadas, el al, Approximate Analysis of Short Circuited Ring Patch Antenna Working at TM01 Mode, IEEE Transactions on Antennas and Propagation, Vol. 54, No. 6, June 2006, combined with a circular patch with amplitude and phase weighting to create a null at the horizon. While the HRN's performance is exceptional with regard to its horizon nulling capability, its cost is relatively high due to the required active electronics. Additionally, the exceptional null of the HRN degrades significantly when installed near other scattering objects, which typically occurs for which happens in most real world installation environments.
Thus, a low cost RHCP antenna with sufficient beamwidth and deep horizon nulls is desired for GPS applications.